1. Field of the Invention
The invention is in the field of heavy duty axle housings useful for large and heavy trucks in which a banjo housing is provided at or adjacent the center of the axle housing to accommodate the differential drive gears and laterally extending tubular members are formed in a rectangular cross-section to withstand torsional and vertical loading strains and stresses and to resist metal fatigue caused by pounding of the vehicle over rutted and potholed roads. The invention also lies in the field of manufacturing methods for axle housings using novel sheet steel material of uniform thickness.
2. Description of the Prior Art
PRIOR MANUFACTURING METHODS--GENERAL
As pointed out in the patent to Riemenschneider, U.S. Pat. No. 1,945,076, rear axle housings for motor vehicles have been manufactured by various methods all of which require a careful selection of the starting stock material, special bending equipment and the employment of skilled operators. Generally, the prior methods contemplate stamping out blanks and shaping them by known bending methods into upper and lower halves of the axle housings which are welded together along longitudinal seams on each side to extend the full length of the housing. Such methods are expensive because a considerable percentage of the stock material is wasted due to the irregular outline of the blank and because of the excessive length of the weld.
The present methods which have been developed to overcome the problems noted in Riemenschneider are the hot forging methods such as those developed by Schneider et al, U.S. Pat. No. 2,674,783 which requires the use of expensive equipment dies and the services of skilled forging operators and welders, while at the same time requiring a blank having an irregular outline which is expensive because of the substantial waste of material.
Further, waste occurs not only in the material lost in cutting or stamping the blank but also takes place in forming the axle housing by hot forging and welding as for example in the Reimenschneider patent above-mentioned wherein a slotted tube is formed from strip stock, then heated and the end of the heated tub placed in a suitable female die to forcibly engage a complementary male die and thereby produce a predetermined upsetting of the blank which is enlarged by additional working to spread the upset. An additional step is required to outwardly weld a part thereof to form a frustoconical portion. That portion is then enlarged into an annular portion which is machined to a smooth finish and provides part of the housing for the axle. The added parts which are removed represent additional waste.
An additional example of a banjo-type drive axle housing having a very short tapered portion made by stamping of relatively thin sheet metal is found in U.S. Pat. No. 4,068,541 to Sakamoto et al. However, in the Sakamoto et al patent, the inner banjo shape adjacent the taper portion is reinforced by welding fixed partitions positioned to double the thickness in regions along the inside of the taper of the casing.
The present invention utilizes technology similar to the hot forming and bending technology of Schneider et al, U.S. Pat. No. 2,674,783 rather than stamping and internally reinforcing. The older efforts of stamping as well as the newer efforts of reinforcing a stamped banjo-shaped axle housing are to be distinguished from hot forming and bending operations which in accordance with the present invention are required to provide a faster fade in and adjacent to the transition zone between the differential receiving portion and the beam arms of the axle housing.
THE PRIOR ART FOR THE BLANK
The prior patent to Schneider, et al, U.S. Pat. No. 2,674,783 shows a blank having an irregular outline which is formed in an apparatus and method for making long tapered or slow-fade banjo-shaped axle housings. The irregular shaped blanks are cut in a special shape as shown in FIG. 6 of the Schneider, et al patent and are hot forged into longitudinal, channel-shaped housing halves having the banjo-shaped differential-receiving region in the center and the tapering beams joined to the wheel spindles at each end. The blank is heated to about 1800.degree. F. The hot forged blanks of irregular design are formed in several stages in dies to provide a circular banjo portion and a long tapered transition zone to the straight axle portions extending to the wheels. Channels of a U-shape are formed. The outer end regions of the channel walls are reduced in height to thicken the metal in the corner regions. The side walls are scarfed at a critical scarfing angle for welding. Each respective half of the banjo-shape axle housing is thus formed in a manner so that two halves will match. The aligned scarfed and mated edges of the U-shaped channels which are matched are then welded together at the scarfed edges.
A variety of sizes of slow fade or long tapered axle housings are commercially made from various sizes of blanks. These different sizes come with the axle arms of the same length or of different lengths so that the differential receiving opening may be centered or off-set to the right or to the left.
RADIUS CHANGES TO IMPROVE SCHNEIDER, ET AL, U.S. PAT. NO. 2,674,783
The present invention is directed to improvement in the torsional resistance of the banjo or differential receiving portion and to strengthening the taper or transition zone between the edge of the circle constituting the differential receiving portion and the axle beam arm portions of a drive axle housing which is otherwise similar to those produced in the Schneider et al process. In the long tapered gradual transition zone which is characteristic of the Schneider, et al product, the axial gradual transition zone distance from the center of the housing to the end of the radius, is about two times the radius R.sub.1 defining the axially extending curvature of the corner sections of the housing and is characterized as a slow fade or transition of that radius R.sub.1 and the radius R.sub.2 defining the curvature of the web into the laterally extending beam arms. Thus the transition zone taper in the conventional manufacture extends along a relatively large distance which is substantially equal in distance to the radius R.sub.1 of the banjo portion. For example, the gradual transition zone of a Schneider et al type of housing with a 9 inch R.sub.1 radius of the banjo portion extends about 8 to 9 inches along the horizontal axis and terminates about 17 inches from the origin or center of the differential receiving portion of the housing. In contrast to this long tapered portion in the slow-fade which merges with the straight beam, the present invention cuts the fade in half. Thus in comparison with the prior art example of a 9 inch R.sub.1 radius having a transition distance terminating about 17-18 inches from the center line passing through the center of the banjo or differential receiving portion, the length of the tapered portion or transition zone is reduced to about 40-48% of the R.sub.1 radius in the present invention.
DIFFERENCES OF THE BLANK OF THE INVENTION OVER THE BLANK IN SCHNEIDER, ET AL, U.S. PAT. NO. 2,674,783
The blank of strip material shown in the patent to Schneider, et al, U.S. Pat. No. 2,674,783 at FIG. 6 shows a number of significant differences in the longitudinal, vertical and internodal dimensions and relative proportions of the arms and nodal portions in comparison with the blank used in the present invention. For example; the width of the blank, i.e., the distance between the nodes in the Schneider, et al blank is 14 inches or about 14% wider than the maximum width or distance between the nodes in the blank of the present invention.
The Schneider et al blank of FIG. 6 is not only wider but a substantially greater length of that blank is dedicated to forming the differential receiving portion and transition zones than with the blank of the present invention. In the Schneider et al blank, about 71% of the length of the blank is dedicated to forming the differential receiving portion and transition zones while 29% of the length is used to form the beam arms at each end of the axle housing. In the blank of the present invention 48-50% of the axial length is dedicated to formation of the beam arms sections of the housing while 50-52% of the axial length is dedicated to formation of the differential receiving portion and transition zones of the axle housing.
Although the width of the blank of the present invention is less than the width of the Schneider et al blank the nodal slopes defining the contour of the top and bottom edges do not cut as deeply into the blank as the slopes of the Schneider et al blank. For example, excluding the depth of the notches and the nodal and node slope portions, the width of the remainder of the Schneider et al blank is about 66% of the maximum width across the nodes wherein the width of the corresponding remainder of the blank of the present invention is 70-71% of the maximum width of the blank. There is greater waste in the portions cut away to form the Schneider et al blank.
A further distinction resides in the nodal slopes per se. In the blank of the present invention, the inner nodal slopes are substantially steeper than the outer nodal slopes. There is no such significant difference between the inner nodal and outer nodal slopes in the Schneider et al blank.
A still further distinction arises from the plurality of notches 24 in Schneider, et al which are cut into the edge of the blank between the nodes at the top and between the nodes at the bottom which is much more complex in Schneider, et al than in the blank of the invention. In FIG. 10 of Schneider, et al, three such notches are shown in the side view of the formed blank and these three cut out notches on each long edge make six notches in total, three on the upper edge between the nodes and three on the lower edge registering with the upper notches. In contrast the present invention has only one notch on each long edge. The blank has only two notches total, which are positioned precisely to locate the opening for the entry of the differential ring gear.
From simple inspection of the drawings of the blank of the invention and the Schneider, et al blank, it is seen that the blank of the present invention both utilizes and wastes less metal than the Schneider et al blank. Moreover, as will be described more fully, the portion of the blank dedicated to forming the differential receiving portion and transition zones is subjected to a more complex and intense bending and hot forming operation which improves the strength and fatigue properties of the finished axle housing.
The movement of the metal dedicated to forming the differential receiving portion and transition zones of the housing is best accomplished in the invention by forming the blank first in the central area before forming the end portions. Thus the flat blank is curved in the center to form wings which extend on each side. This is a semi-formed half with flat wings which represents an intermediate stage of manufacture in the first step of the forming process of the present invention. The wing portions are bent into U-shape and these steps are accomplished in a single first bending operation which precedes a single sizing or straightening operation. Generally, the apparatus of Schneider, et al, may be used for these steps. The usual procedure of Schneider, et al, of heating the blank material to a temperature of about 1800.degree. F. in order to maintain the proper temperature condition for bending need not be followed and it is preferred to operate at lower temperatures to reduce the amount of decarburization at the surface of the axle housing halves and reduce wear of the forming die material.
It is a characteristic of the method of the present invention that the forging operation is carried out in a highly intensive manner in several stages, including a first stage above 1350.degree. F. for hot bending and a second stage where the walls are squared off and the corner radii are sharpened. The modified fast-fade area is carefully sized during formation of the banjo portion which takes place in the highly intensive first stage of bending and forming. The taper or transition of Schneider, et al, extends over twice the distance as compared with the extension of the taper or transition in the present invention. This requires highly intensive and accurate bending of the transition portion of the blank of the invention. The length of the transition zone along the longitudinal axis from the edge of the circle of the banjo is only about 45% of the radius R.sub.1 in the present invention instead of 100% of the radius R.sub.1 as in conventional axle housings produced by the Schneider et al process.
The present axles are "hot formed" i.e. bent in the die, and the blanks are heated to temperatures above 1350.degree. F. and below about 2100.degree. F., preferably about 1400-1650.degree. F. During forming the blanks are bent in the dies under conditions in which there is no attempt to move or displace the metal as in a hot forging operation conducted at temperatures greater than 2100.degree. F.
There are essentially three different styles of axle housings, a first style in which both of the axle beam arms are of the same length with the bowl in the center, a second in which the left beam arm is shorter than the right beam arm and a third in which the left beam arm is longer than the right beam arm. In the drawings of the present invention the shorter left beam arm is shown as one example but the invention can be carried out with all three styles.
The present invention provides for lower inventory requirements for various blank sizes and blank styles (left, center or right) despite a higher working requirement in forming and bending and a significant increase in strength and resistant to breakage is achieved in the product.
The modified fast fade design of the present invention provides the following advantages:
1. Savings of metal for an axle of a specified load rating, i.e. less weight, less waste and less expense.
2. Stronger axle; even though less metal is used for the entire axle more metal is provided and structurally formed to increase the load carrying capacity of the transition zones and the area adjacent the transition zones over the load carrying capacity attained with other designs.
3. The axles are more easily packagable in the vehicle:
(i) fast fade minimizes width of curved differential receiving "bowl" portion and increases horizontal length of beam arms thereby providing greater lateral clearance for depending truck body frame members on each side of differential bowl, and PA1 (ii) increased length of beam arms permit location of truck suspension to axle brackets over a greater horizontal distance on each side of the differential bowl.
CHARACTERISTICS OF MODIFIED FAST FADE
A new axle housing is provided by purposely forming a shortened banjo section thereby providing a transition zone only 40-48% as long as the transition zone in the conventional method. The short neck which results from a shortened transition zone from the circular portion to the beam arm portions of the housing is less than 1/2 that of the transition zones provided by conventional forging operations. Thus for a 9 inch R.sub.1 radius of the differential receiving portion of the axle housing the transition zone extends only an additional 4 inches to where it blends smoothly into the bearm arms.
To describe the modified fast fade axle housing and the transition zone where the differential receiving portion blends into the beam arms, a first radius R.sub.1 centered at the origin defines the locus of the center points of the radii (R.sub.c) forming the corners joining the legs to the webs of each U-shaped section and a second eccentric radius R.sub.2 measured from an eccentric point of origin O' defines the outer wall curvature or curvature of the web forming the outer wall of the housing, so that the distance between the true origin O and the eccentric origin O' and the length of the second radius are factors which reflect a substantial increase in displacement of the web or outer wall of the housing relative to the origin O about the center of the differential receiving portion of the axle housing. The 9 inch R.sub.1 radius and 0.81 inch displacement of an eccentric radius of 10.50 inches provides an axle housing having outstanding strength and resistance to stress factors and of less weight in comparison to conventional axle housings rated for the same load carrying capacity which are produced using the long taper rather than the short taper or short transition zone. The corner radius is 0.62 inch at each end of the housing. The web or outer wall of the housing defined by the R.sub.2 radius (10.5 inch) is blended into the web or outer wall of the beam section with a 5 inch blending radius and the locus of the corner radii defined by the radius R.sub.1 (9 in.) is blended into the locus of the corner radii of the beam arms with a 2 inch blending radius.
The short throat transition rather than a long transition represents a substantial shortening of the taper in the throat portion of the axle housing and corresponding savings of weight and metal. The radially displaced web or outer wall provided by the eccentric radius R.sub.2 and displaced origin O' increase the vertical dimension of the U-shaped channel from the horizontal center line through the transition zone and provide a greater load carrying capacity to that section of the axle housing. This is desirable since the circular apertures provided to each major face of the differential receiving portion of the axle housing are closed on one side with a bowl-shaped cover welded to the axle housing and on the other side with a differential carrier bolted to an annular ring welded to the axle housing. The continuous annular weld joints securing these members to the axle housing are structurally sound, however the stresses resulting from the thermal gradient at the radially outer periphery of the weld joint may reduce the load carrying capacity of a vertical section through that part of the housing by as much as 25%.
The deleterious effect of such a reduction in strength is overcome by tne present invention where each vertical section through the radially outer zone adjacent the circular weld joint is increased in height, i.e. spaced outwardly from O and upwardly from the horizontal weld joint to insure that the load carrying capacity of each vertical section is greater than some predetermined value; e.g. greater than the load carrying capacity of the beam arms.